Data Consumption

The data consumption of Android Nextcloud Client 3.6.0 is very high. (400 MB in the first week of May) That was different before. I have one auto-upload directory which is only synchronized via WiFi. During this time exactly one image file (<10 MB) was uploaded.
I’m not sure what and when is synchronized and I don’t know where to find more detailed information.
I have now switched off the synchronization and then the data consumption seems to stagnate.
Any suggestions what I can do to find out, what is wrong here?